Figure 1-6. Rapid growth, dense cover, and soil-binding 

 roots with nitrogen-fixing bacteria, make kudzu (Pueraria 

 thunbergiana) valuable in erosion control in the south- 

 eastern United States. October, Mississippi. (U.S.D.A. Soil 

 Conservation Service.) 



The influence, or reactions, of a species upon the habitat may 

 produce optimum conditions for the same or for other species. 

 Cattail (Typha), for example, when it first invades a lake improves 

 the habitat for itself, but later, as the soil level is raised, the 

 habitat becomes more suitable for invaders. Cycles of changes 

 may occur (see Chapter 4) as in Great Britain with Festuca ovina as 

 the key plant. It invades stony areas, improves the soil, and forms 

 hummocks, which gradually increase in height causing the fescue 

 to lose vigor. Lichens then invade and erosion follows, resulting 

 in a stony surface again. The cycle includes a building phase and 

 a degenerative phase, in which the reactions of the plants change 

 the environmental conditions from poor to good and then to poor 

 again for the respective species. These changes and the differences 

 in ecological amplitude make the cycle possible. ^^^ 



Some species have more pronounced reactions than others, and 

 this may be of considerable importance in soil improvement 

 (Figure 1-6). In Nebraska the cool-season grasses: Agropyron 



Ecologfical Characteristics of Species & Populations • 21 



